1. Field of the Invention
Embodiments of the invention relate generally to the field of materials. More particularly, embodiments of the invention relate to methods of controlling formation of a segregated phase domain structure within a chemical reaction product, compositions of matter including such a segregated phase domain structure, and machinery having a complex tool relief for making such compositions.
2. Discussion of the Related Art
Prior art copper indium selenide based photovoltaics, sometimes called CIS based PV, are known to those skilled in the art of solar cells. CuInSe is the most reliable and best-performing thin film material for generating electricity from sunlight. A concern with this technology is that raw material supply constraints are going to arise in the future as the production of CIS PV increases. For instance, indium does not occur naturally in high concentration ores. Typically, indium is obtained from the discarded tailings of zinc ores. As the production of CIS PV approaches the large scale range of from approximately 10 gigawatts/year to approximately 100 gigawatts/year, indium supply constraints will become manifest. These supply constraints will lead to increased costs. Further, as the production of CIS PV increases, other raw material supply constraints will also emerge. What is required is a solution that reduces the amount of raw materials needed per watt of generating capacity in CIS PV thin films.
One approach to reducing the amount of raw materials needed is to reduce the thickness of the CIS PV thin film material. The inherent absorption coefficient of CIS is very high (i.e., approximately 105 cm−1). This means that most of the incident light energy can be absorbed with a very thin film of CIS. The use of a back surface reflector can further reduce the thickness necessary to absorb most of the incident light energy. While prior art CIS PV products are typically at least about 2 microns thick, it is important to appreciate that 0.25 microns is theoretically sufficient for a CIS PV thin film located on a back surface reflector to absorb most the incident light energy. What is also required is a solution that produces thinner CIS PV thin films.
Meanwhile, field assisted simultaneous synthesis and transfer technology has been developed that is directly applicable to the manufacture of thinner CIS PV films. Various aspects of this field assisted simultaneous synthesis and transfer technology (which aspects may or may not be used together in combination) are described in U.S. Pat. Nos. 6,736,986; 6,881,647; 6,787,012; 6,559,372; 6,500,733; 6,797,874; 6,720,239; and 6,593,213.
An advantage of field assisted simultaneous synthesis and transfer technology is that it works better as the precursor stack becomes thinner. For instance, the vapor pressure of selenium in a CIS based reaction product layer is a function of temperature. The pressure needed to contain the selenium is a function of the temperature required for the process reaction. It is important to appreciate that the voltage, if utilized, to achieve a desired pressure goes down as the thickness goes down. As the required voltage is reduced, the physical demands on the system (e.g., stress on the dielectric) go down. Therefore, as the precursor stack is made thinner, the voltage needed to generate a given pressure goes down; which reduces stress on the dielectric (for instance a release layer), thereby expanding the scope of materials that can be utilized as a dielectric.
Another advantage of field assisted simultaneous synthesis and transfer technology is that it enables a lower thermal budget. The lower thermal budget is a result of higher speed of the field assisted simultaneous synthesis and transfer technology compared to alternative approaches such as (physical or chemical) vapor deposition. In addition to the time and energy savings provided by the field assisted simultaneous synthesis and transfer technology, the quality of the resulting products can also be improved. For instance, in the case of manufacturing CIS based PV, the lower thermal budget enabled by the use of field assisted simultaneous synthesis and transfer technology leads to the reduction of undesirable reactions, such as between selenium and molybdenum at the interface between the CIS absorber and the back side metal contact. The reduction of this undesirable reaction results in reduced tarnishing which in-turn results in higher back surface reflectivity.
Recently, it has been demonstrated that CIS thin films made by conventional techniques contain domains resulting from fluctuations in chemical composition(1-2, 5). Undesirable recombination of charge carriers takes place at the boundaries between the nanodomains within such a CIS based PV absorber. Therefore, what is also required is a solution to controlling, and ideally optimizing, the boundaries between, these nanodomains with varying chemical compositions.
Heretofore, the requirements of reduced raw materials requirements, reduced thickness and controlled boundaries between nanodomains referred to above have not been fully met. What is, therefore, needed is a solution that simultaneously solves all of these problems.